1. Field of the Invention
The present invention relates to a lens driving device for moving a lens for focusing by the driving force of a vibration motor wherein a movable member is driven by vibrations generated in a vibrator by applying a periodic voltage to an electromechanical energy conversion element thereof, such as an electrostrictive element or a piezoelectric element.
2. Description of the Related Art
Methods for adjusting the speed of various types of motors by changing the frequency of a periodic voltage are known.
To set the motor at a desired speed by the above-mentioned methods, a frequency corresponding to the desired speed is selectively applied to the electromechanical energy conversion element. However, such methods have a drawback in that the motor starts rotation rapidly. Also, since the relation between the speed and the frequency is readily affected by the ambient temperature or other environmental conditions, the speed of the motor may change depending on the environmental conditions.
Accordingly, a method for rotating the motor smoothly and for driving the motor at a stable speed independent of changes in the environmental conditions has been proposed. In this method, the drive frequency applied to the motor when the motor is activated is gradually reduced from a high frequency. After it is confirmed that the motor has been activated, the actual rotational speed of the motor is compared with a desired rotational speed. As is known in the art, in an operable range, the driving frequency and the motor speed have an inverse relationship. Thus, when the actual rotational speed is higher than the desired rotational speed, the applied frequency is increased so as to obtain the desired speed. When the actual rotational speed is lower than the desired rotational speed, the frequency is reduced so as to obtain the desired speed.
The motor can be accelerated smoothly to a desired speed in a short period of time by setting the initial driving frequency at a high frequency at which the motor actually starts rotating, and by gradually reducing the frequency from that set frequency. However, the characteristics of the motor are not fixed, i.e., the characteristics of the motor are affected by changes in the environment, such as changes in the ambient temperature. Therefore, when the frequency at which the motor is activated is fixed, it may not be possible to accommodate changes in the ambient temperature or the like. As a result, it may take a longer time for the motor to be activated, or the motor may be activated to rapidly. In the worst case, it may not be possible to activate the motor at all.
In order to solve the above-described problems, Japanese Patent Laid-Open No. Sho 63-209481 has proposed a method in which the frequency at which the motor initially started rotating in the previous drive operation is stored, and in which scanning of the driving frequency is initiated in a subsequent drive operation from that stored frequency toward a lower frequency. In this method, however, it is possible that the motor may not be activated even when frequency scanning is performed in a direction in which the frequency is reduced. In order to avoid such a worst case, scanning may be repeated again starting from the highest frequency of the set frequency range.
When the torque of the motor is small, a helicoid ring is generally rotated through a gear train between the output of the motor and a driving portion of the focusing lens. Such a drive device requires a lens control accuracy of within a few microns, and a lens drive detection mechanism capable of detecting the movement of a lens at a high resolution of within few microns. The lens drive detection mechanism is generally provided at the first stage of the gear train. In the mesh of the gears, a backlash inevitably exists. Therefore, even if the lens drive detection mechanism detects that the lens drive value (the detected number of pulses) has reached a predetermined value representing an amount of lens movement, the actual lens movement may be smaller than that predetermined value by a value corresponding to the backlash of the gears. Furthermore, when when fine adjustment of the lens is desired, that is, when the lens is to be driven through an amount substantially corresponding to the backlash, it may not be moved at all even when the motor is driven.
To solve the above-described problems, various methods have been proposed. Examples of such methods include a method in which the lens is driven by a value that is increased by a number of pulses corresponding to the backlash, and a method in which the detection operation of the lens drive detection mechanism for detecting the drive of the lens is suspended while a backlash exists.
However, in the above-described conventional methods, since the motor is accelerated in the same manner regardless of the existence or non-existence of a backlash, when backlash exists, the motor may be accelerated to a speed higher than the intended speed due to a reduced load of the motor. Therefore, where the lens is to be driven to effect a fine adjustment, the motor may reach the desired stop position before it is sufficiently decelerated, and it may overshoot the desired stop position. This necessitates repeating the drive operation of the lens several times before the lens is focused. In the worst case, the lens may be moved back and forth past the desired stop position (this is called a hunting) and it may not be possible to focus the lens at all.